Industrial recorders using roll chart paper have been used to monitor process variables for many years. Typically these recorders have contained roll charts six to seven inches wide to accommodate multiple process measurements, positioned so that the just-completed chart record extends across the front face of the device. In order to change a roll, the instrument first is pulled out partially from the control panel. Upon opening the front face of the instrument, the completed roll chart is removed and a new supply roll is inserted into a mating receptable. A small amount of chart paper is then unwound from the supply roll and attached by tape or otherwise to a take up spool located directly below and parallel to the supply roll. Thus, the removal and replacement of roll charts for such prior art devices is accomplished with the recorder being essentially in situ.
As processes have become increasingly complex, there has been greater need to integrate and display process information more effectively for control room operators. Control rooms typically display such data on large panels (fifty feet or more in length), and the total cost per square foot associated with these control panels is quite high.
Accordingly, the size of process instruments has been drastically reduced in the past few years to increase the amount of process information displayed for a given control panel layout. For example, the width of industrial recorders has in recent years been cut nearly in half. Of course, such dimensional modifications require significant design changes in chart paper feeding mechanisms and display techniques. Along with changes in the mechanical design, there has also been parallel efforts to reduce the electrical energy consumption of the instruments, thereby resulting in the use of relatively low-power motors to drive both the pens and the chart record.
In certain of the process recorders, the entire chart record is contained on a removable unit commonly referred to as a cassette. Such a unit is advantageously positioned within the instrument so that a sizeable length of just-completed chart record extends along one entire side wall of the instrument, thereby increasing the amount of historical trend data which an operator can view when the recorder is partially withdrawn from the panel.
To simplify the instrument design, it is preferable that the cassette be arranged so that, when the chart record is due for replacement, the cassette is removed entirely from the instrument before carrying out the procedure incident to replacing the chart. This has created a special problem since the control room typically does not have a readily-available, close-by work bench or the equivalent on which the cassette can be placed while being worked on to install the replacement chart record. Since control room operators often monitor numerous complex, critical processed by themselves for long periods of time, this chart replacement procedure must be accomplished in a minimum amount of time. Thus the overall cassette arrangement, and the replacement roll chart to be installed therein, must be designed to permit installation with one hand while the cassette is being held in the other hand, while the control room operator is still able to keep track of various process conditions.
Still another problem frequently presented in roll chart recorders of the cassette-type is the possiblity of paper jams due to misalignment between the paper roll and the paper feed mechanism. This problem is aggravated by the relatively long distances over which the paper is fed and also by the energy-efficient, low-power chart drive systems which often cannot overcome even the slightest irregularities associated with paper rewind. To some extent, this situation can be alleviated by providing mechanical paper chart guides at strategic places along the feed path. However, such guiding devices do not present a perfect solution to the problem, and moreover they increase the overall dimensions of the equipment which is especially undesirable in modern applications requiring instruments with minimum size.